repo: Delete the last use of GFile tmp_dir

The keyring isn't large, so let's just fall back to copying it
rather than requiring `renameat()`.

Prep for `ostree_repo_open_at()`.

Closes: #821
Approved by: jlebon

src/libostree/ostree-gpg-verifier.c

@@ -116,7 +116,7 @@ _ostree_gpg_verifier_check_signature (OstreeGpgVerifier  *self,
   if (result == NULL)
     goto out;
 
-  if (!ot_gpgme_ctx_tmp_home_dir (result->context, NULL,
+  if (!ot_gpgme_ctx_tmp_home_dir (result->context,
                                   &tmp_dir, &target_stream,
                                   cancellable, error))
     goto out;